Do not generate Application.kt when interfaceOnly=true

[tobilarscheid]

In an effort to make kotlin-spring behave the same as spring (java), I disabled generation of the Application.kt when interfaceOnly=true. This is similar to what spring does: https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/SpringCodegen.java#L368

Fixes #5165

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[tobilarscheid]

./mvnw test is green locally, any ideas why circleci is failing?

[wing328]

We're seeing the same issue related to the Dart client in the master. Please ignore it for the time being.

[wing328]

LGTM but I've not tested it locally.